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Kerberos uses des-cbc-hmac (mac and encrypt) and rc4-hmac-exp based on md5 (mac and encrypt I think) in their encryptions.

Those encryptions are considered as weak and should not be used. Why are they considered as weak? What attacks can be launched against those encryptions?

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Kerberos uses des-cbc-hmac (mac and encrypt) and rc4-hmac-exp based on md5 (mac and encrypt I think) in their encryptions.

Those encryptions are considered as weak and should not be used. Why are they considered as weak? What attacks can be launched against those encryptions?

Regarding the ciphers:

  • DES is considered to be a weak block cipher because the key length is relatively short. This is one reason why "triple DES" (3DES) was created.
  • RC4 is a stream cipher that suffers from a variety of different vulnerabilities detailed, for example, on the RC4 Wikipedia page.

Regarding the cipher mode: Cipher block chaining (CBC) is not easily parallelizable, so other modes like CCM or GMC might be preferred.

Regarding authenticated encryption: It is usually accepted that "encrypt then MAC" is better than "MAC then encrypt" and both are better than "MAC and encrypt." See, for example, section 5.3 of Introduction to Modern Cryptography (3rd Edition) by Katz and Lindell. Those authors use the terms "encrypt-and-authenticate" vs "authenticate-then-encrypt" vs "encrypt-then-authenticate."

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  • $\begingroup$ I see. But does not the authenticated encryption protect it from these attacks? $\endgroup$
    – adi
    Sep 28, 2021 at 19:27
  • $\begingroup$ Which attacks? For example, the DES block cipher has a weak/short key length regardless of the mode and regardless of the authenticated encryption scheme. $\endgroup$
    – hft
    Sep 28, 2021 at 20:52
  • $\begingroup$ I see, so the attacker can read the data. Same thing for the rc4. But can he change the data in some way - does not the mac protect it? $\endgroup$
    – adi
    Sep 29, 2021 at 5:03

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